Direct-to-Satellite Connectivity for IoT: Overview and Potential of Reduced Capability (RedCap)

The satellite internet-of-things (SloT) is growing at an impressive speed and promises to offer ubiquitous wireless coverage for applications in remote regions. The recent developments confirm some 3rd Generation Partnership Project (3GPP) and non-3GPP radio access technologies (RATs) could potentially support SloT services. Among the non-3GPP technologies, Long Range (LoRa) is one of the prominent and proven RAT for SloT services. The real flight-test experiments confirmed the feasibility of direct communication between terrestrial LoRa end device and low Earth orbit (LEO) satellite enabling Direct-to-Satellite (DtS) connectivity. However, the state-of-art publications extensively investigate the LoRa DtS scalability and revealed that the communication performance degrades due to co-channel interference. To overcome this challenge, the new variant of LoRa, namely Long Range-Frequency Hopping Spread Spectrum (LR-FHSS) implies frequency hopping, header diversity, and low code rates that make it highly robust against co-channel interfer-ence and noise. Similarly, Eutelsat satellite operator is working on Sigfox for SloT services. In 3GPP scope, the narrowband loT (NB- loT) is another potential candidate for implementing DtS connectivity. MediaTek and Inmarsat have conducted field test experiments that confirmed the feasibility of NB- loT DtS connectivity. More recently, Reduced Capability (RedCap) was introduced in 3GPP Release 17 for 5G new radio (NR). The RedCap features the functionalities between the NB-IoT, Sigfox, LoRa, and 5G NR user equipment (UE). As of today, we are not aware of any work discussing the practicalities of RedCap D$t$S. In this paper, we present a technical overview of satellites in 3GPP, and RedCap technology and identify and highlight the relevant challenges associated with RedCap DtS connectivity. We also mention several research directions to further investigate RedCap DtS connectivity.